I think this has little to do with Limesurvey, but rather with your email approach and/or Yahoo and their users.

From what I understand, there have been complaints by users for your IP. I assume that someone marked your emails as spam. You should check with the respective users.

Make sure that your IP is not within a 'bad neighborhood' as well. In our company, if we receive spam or unwanted emails and report them, your email server admin will block the IP or even the whole IP range, if it is suspicious.

I think the explanation in this link is fairly clear. There are a few things that can easily lead to being blocked:
- You are sending a lot of emails (there are always emails that are old or incorrect, if an email provider like Yahoo receives a lot of emails where no account exists, or if a few users mark your email as spam, you can be easily blocked). Make sure that your contact data is very accurate and up to date.
- Your server is located in Bulgaria, which does not have the best reputation when it comes to spam/scam emails.
- Depending on your text you might also trigger some other things, depending on language/words used
- You are hiding your identity (makes it even more suspicious)
- Your email server might not be configured perfectly (SPF, etc.)

Not getting labeled as spam is a science in itself, when sending mass emails.

Oath (Yahoo, AOL and a few others) is know for it's "chaotic" blocking routines. You can get blocked, when you send not enough emails over time

Most common issue is people marking the email as SPAM instead of unsubscribing.
Feedbackloops can be created to get an email when people clicking SPAM on your mails. That way you can remove them from your lists to minimize SPAM labels.

It make sense, when you think about the primitive routines used to block mailservers. A mailserver which is suddenldy sending a higher volume is labeled as a hacked server which is sending SPAM.
When sending newsletters you can send volume over time. When just sending surveyinvitations (e.g. after a yearly exhibition only) you can't get the same volume over a longer period of time.
The industry is a getting a bit silly. On the one hand they want you to use your own mailserver with a dedicated IP address to recognize sender streams more easily. On the other hand they want you to send a constant flow of emails without much change in volume.

And the threadstarter is not haven a constant flow of emails.
Removing blocks will be part of the game with Oath.

If you're doing survey maling for companies people don't recognize the sender as legit. So getting blocked more easily on webmail providers is not unusual.